1. Technical Field
The present exemplary embodiment relates to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excites nuclear spins of an object magnetically with an RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation and, more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to suppress or reduce influence of respiratory body motion on imaging.
2. Description of Related Art
Magnetic Resonance Imaging is an imaging method which magnetically excites nuclear spins of an object set in a static magnetic field with an RF signal having the Larmor frequency and reconstructs an image based on an NMR signal generated due to the excitation.
In MRI imaging of the upper abdominal area such as a liver, a fluctuation of an image and an artifact called ghost where the outline of a body surface would be thinly overlapped occur due to respiratory body motion. To avoid these, there is the conventional technique for reducing the influence of respiratory body motion by sorting an encoding order in accordance with a breathing period.
Further, suppression of image degradation is performed by alleviating the object's breathing movement by means of the auto voice function to stabilize a TR (repetition time) (see, for example, Japanese Patent Application (Laid-Open) No. 2001-346773).
When imaging is performed while holding the breath, even if a parallel imaging that is a high-speed imaging technique for acquiring signals with multiple surface coils is applied, an imaging time is limited to 10-30 seconds. An imaging time here becomes TR×(a matrix size in a phase encode direction). Therefore, when imaging is performed while holding the breath, it is difficult to improve a matrix size in a phase encode direction, i.e., a resolution. Moreover, since imaging time is short, SNR (signal-to-noise ratio) is also limited, and imaging with a high SNR and a high resolution for examining detailed disease states is difficult.
Meanwhile, when imaging is performed with the conventional breathing-compensated method, a large difference occurs in image quality of obtained images depending on the stability of a heart rate of the object. This means if both a breathing level of the object and a breathing period stay constant, an ideal synchronous imaging can be performed. However, when breathing of the object becomes erratic, disturbed amplitude and phase because of breathing exist in acquired data even if the conventional correction in a body motion is performed and, therefore, this disturbance appears as an image artifact.